You have different canals in your ear, mostly BPPV happens in the posterior, but could be in another.
Yes, it's possible to have vertigo when you're lying on your back or left side and not on the right or standing or tilting..depends where the crystals are stuck. You could be having a sludge moving back and forth and not a crystal, and it could be stuck to the cupula and not in the canal.
or...you're having it in your other ear.
Feeling light headed when sneezing seems plausible....it could be that you have eustachian tube dysfunction at the same time, or when your head moves sharply, your brain is picking up the movement and compensating for the slight impairment from your canals..
What ear were you originally diagnosed in?
I would suggest you do the Epley yourself. If you know which ear (I'm assuming the left since you react to the left), have your head turned to the left to begin with.
I would also suggest you take Sudafed and see if that helps. I use the store-brand pseudephedrine 60mg tabs and cut them in half..or if I'm having an attack, I take a whole.
As well, using a Neti Pot to help keep things in your sinuses clear will help.
When I'm "off", my vision can affect to feel light headed. It's related to the BPPV, not another issue...but all works together regarding messages sent to the brain.
You could also check out the Semont maneuver...if you have the crystals stuck on the cupula, that works well.
Hope you feel better soon.
It's all a matter of perspective!